Chlamydial infections cause trachoma, pneumonia and urogenital diseases in humans. Many more asymptomatic carriers have persistent chlamydial infections that pose a significant public health problem. These obligate intracellular pathogens evade normal host responses to infection by forming inclusions that perturb cellular vesicle trafficking. While their intracellular replication is a barrier to horizontal gene transfer, comparative genome sequence analysis indicates that they have acquired new genes since their evolutionary divergence from non-pathogenic Parachlamydia sp. UWE25. This project assesses the contribution of several acquired genes to chlamydial persistence. Eight chlamydial genomes encode a conserved cluster of four uncharacterized genes not found in the UWE25 genome. This cluster includes a homolog of a pyruvoyl-dependent arginine decarboxylase gene, previously identified in the non-pathogenic archaeon Methanococcus jannaschii. Adjacent to that chlamydial gene is a homolog of the Pseudomonas aeruginosa arcD gene that encodes an arginine transporter. Together, these two genes could constitute an arginine uptake, decarboxylation and agmatine export system in the Chlamydiales. Reducing arginine and increasing agmatine concentrations in the host cell is expected to increase pH, inhibit polyamine biosynthesis and inhibit the activity of inducible nitric oxide synthase, a central regulator of host signaling pathways. A biochemical approach is proposed to test the hypothesis that the decarboxylase and transporter proteins function coordinately to consume host-derived arginine. Because homologous proteins frequently catalyze similar reactions using different substrates, these experiments are necessary to distinguish among alternative functions. Alternative roles for the decarboxylase include histidine decarboxylation to histamine (an important immune response regulator) or ornithine decarboxylation to putrescine (a polyamine precursor required for cell proliferation). Therefore we will heterologously express both proteins and assay their predicted catalytic activities. Understanding these genes' functions will address how the Chlamydiales have evolved to circumvent host immune responses and to establish persistent infections. Results from these experiments will support future microbiological studies of the role of this system in chlamydial infection and survival in the host cell. [unreadable] [unreadable] [unreadable]